Yeah, always wait on the PC portion of the system. Seems like prices are always coming down. At a minimum, the prices aren't likely to go UP.
I was just wondering if you were getting one of those fancy arcade LCDs (which, you're not, so forget half of what I wrote). I think you can hook them up like the old CGA arcade stuff, and hence have all the same 15khz hassles, LOL. You don't have to of course... but you could. They're also really expensive, but if you some day bought an arcade mobo, you could wire up a JAMMA harness. All stuff sane humans probably aren't interested in doing

I'm no arcade purist. I have a Neo Geo cab and an extra computer. Makes the arcade far more useful to me, without harming the original equipment. Most of my gaming is on the big screen with a controller...
Whatever you end up building, the ATI chipsets, in general, just seems to work better for Arcade emulation, if these forums are any indication. I'm running NVidia in everything I own, and have had some headaches other people don't seem to have.